Nordintown Blog

Rubbish musings, projects and hackerings from Nordin

Space radio #2 - Space TV

Apr 092015

I am not feeling great at all today and so I took a rare sick day from work and oh look, radio.

I needed to avoid spreading the dreaded lurgie and also to recoop from 2-3 nights of horrible sleep. Rather than sitting back and letting Netflix wash over me, I have resolved to finishing my quadrifilar helix (QFH) antenna project in between coughs. A QFH antenna is a 4 part (hence quad) helical receiving antenna design well suited to the capture of APT (Automatic picture transmission) images from orbiting satellites. I want to make something that I can modify for use with both NOAA/ISS APT/SSTV transmissions, which have their downlinks set to 137.500ish and 145.800Mhz respectively.  

John Coppens (ON6JC/LW3HAZ)[2] puts it better than me:

"The QFH is an excellent antenna for satellites, as it receives from 'the entire northen hemisphere', from horizon to horizon, with all the sky in between. Such an antenna cannot have any significant 'gain', as it doesn't have directivity. Such an antenna is not useful for hot spots, as you probably won't have any clients 'in the sky'."

This design [1] looks to be a winner and so I have brought all my parts and tooling together in my study to hammer out something that I can leave connected to an RTL-SDR/Raspi in my loft. I'll take pictures as I go and aim to get it ready for the weekend ISS passes. To achieve this, I need to:

  1. Check the calculations for the two cable loop lengths, spur lengths, height and distance apart using the target wavelength and core thickness [3]
  2. Mark, drill and cut upon my large and small spur pipes using the measurements obtained; and
  3. Solder the wires of correct length to variboard in the correct way.

All doable, so lets go.

Given the wire I am using (1mm wire stripped from a 3 core electrical cable), the calculations turn out to be: (145.800Mhz137.500Mhz)

Large cable loop: 2223 mm / 2357.1 mm (Brown wire)

  • Antenna height (H1) = 672.9 mm 713.4 mm
  • Internal diameter (Di1) = 295 mm312.9 mm
  • Horizontal separator (D1) = 296 mm 313.9 mm
  • Compensated horiz. separation (Dc1) = 290 mm307.9 mm

Small cable loop: 2112.5 mm / 2240 mm  (Blue wire)

  • Antenna height (H2) =  639.5 mm / 678 mm
  • Internal diameter (Di2) = 280.3 mm / 297.3 mm
  • Horizontal separator (D2) =  281.3 mm / 298.3 mm
  • Compensated horiz. separation (Dc2) = 275.3 mm / 292.3 mm

Diag 1 shows where measurements map to on the design.

Diagram 1, Copyright © 2015 John Coppens [3]

The source website also includes a very handy template that is generated from diameters of the larger and small tubes given. [Diag. 2].

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Diagram 2, Copyright © 2015 John Coppens [3]

I didn't use this as I am drilling holes for both NOAA and  ISS SSTV frequencies. For now I am wiring for 137.500Mhz (NOAA) since I can test it today. All going well, tomorrow I'll cut lengths for the 145.800Mhz ISS SSTV and swap out the cables, moving the separator rods as needed.

I am using 10mm 25mm PVC piping for the horizontal post and old 12mm fibreglass tent poles for the separator rods.  Let the cutting commence.

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Not bad. Not great, but not bad. Next up is the wiring. I used polymorphic thermoplastic to neatly bind the wire to the middle separator rods. The wire can be easily adjusted to make the rounded form required whilst holding firm.

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The polarisation does matter here, so with the smaller loop (green) running north-south, the wire is twisting counter-clockwise. Soldering next [diag 3].  

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Diagram 3, Copyright © 2015 Akos Czermann [4]

A little clumsy, but eventually sorted.

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Having made the basic structure and securely wired it, I need to place it somewhere with a good view of the horizon to test against my store-bought Sky Scanner Rx antenna.

The plan is to hang it in the loft and wire it into a Raspberry PI running RTL-TCP via an RTL-SDR dongle.

Results will follow.

[1] http://www.g4ilo.com/qfh.html 

[2] http://www.jcoppens.com/ant/qfh/index.en.php 

[3] http://www.jcoppens.com/ant/qfh/calc.en.php 

[4] http://sdrformariners.blogspot.co.nz/2013/08/weather-satellites-antennas.html 

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